1. Field of the Invention
The present invention relates to an outboard motor provided with an internal combustion engine including a vertical crankshaft, a pair of intake valves for opening and closing a pair of intake openings opening into a combustion chamber, a valve-operation mode changing mechanism for pausing the opening closing operation of one of the intake valves, and a fuel supply system for supplying a liquid fuel into an air intake passage connected to the pair of intake openings.
2. Description of the Related Art
A prior art outboard motor provided with an internal combustion engine having a valve-operation mode changing mechanism for changing the operating mode of a pair of intake valves according to the operating condition of the internal combustion engine is disclosed in JP-A-186516/2000. This prior art outboard motor is provided with an internal combustion engine having a vertical crankshaft, a valve train, and a fuel injection valve. The valve train includes a camshaft provided with cams respectively having different lifts, a pair of intake rocker arms for transmitting the rotation of the cams to a pair of intake valves placed in intake ports, a pair of exhaust rocker arms for transmitting the rotation of the cams to a pair of exhaust valves placed in exhaust ports, and a power-cutoff mechanism for cutting off power transmission from the cams to the intake rocker arms. The power-cutoff mechanism enables the valve train to operate the pair of intake valves selectively in one of four operating modes.
A prior art internal combustion engine disclosed in JP-A-94433/1992 has a pair of intake valves for opening and closing a pair of intake ports, a valve-operation mode changing mechanism capable of substantially pausing the opening and closing operation of one of the pair of intake valves, and a fuel injection valve. In this prior art internal combustion engine, swirls are generated in the combustion chamber to promote combustion for improving combustion efficiency while the opening and closing operation of one of the intake valves are made to pause.
Incidentally, a valve-operation mode changing mechanism capable of controlling the valve train of an internal engine included in an outboard motor such that the opening and closing operation of one of the pair of intake valves are made to pause substantially may be employed to improve combustion efficiency by generating swirls in the combustion chamber of the internal combustion engine in order to improve fuel consumption through lean-burn combustion. When such a valve-operation mode changing mechanism is employed, the reduction of the quantity of fuel adhering to a part of the internal combustion engine around the intake port closed by the paused intake valve is important for stable combustion.
While the internal combustion engine is operating with one of the intake valves held in an idle state, the fuel injected by a fuel injection valve adheres to a wall around the intake port closed by the intake vale held in an idle state and, although a part of the fuel enters the combustion chamber through a gap formed between the wall and the slightly opened intake valve, a large quantity of the fuel stays on the wall around the intake port.
When the quantity of the fuel that stays on the wall surface of the intake port (hereinafter, referred to as xe2x80x9cquantity of the stagnant fuelxe2x80x9d) increases, air-fuel ratio is liable to vary according to the quantity of the stagnant fuel, making combustion unstable. When a lean-mixture of a higher air-fuel ratio is used for lean-burn operation, the variation of the air-fuel ratio affects the stability of combustion greatly.
The present invention has been made in view of the foregoing circumstances and it is therefore an object of the present invention to provide an outboard motor provided with an internal combustion engine having a pair of intake valves and capable of pausing the opening and closing operation of one of the pair of intake valves, and of reducing the quantity of the stagnant fuel on the surface of an intake port provided with the paused intake valve to suppress the variation of the air-fuel ratio and to improve the stability of combustion.
With the object in view, the present invention provides an outboard motor provided with an internal combustion engine comprising: a vertical crankshaft; a cylinder head provided, for one combustion chamber, with an intake port terminating in a pair of intake openings, and an exhaust port terminating in exhaust openings; a pair of intake valves for opening and closing the pair of intake openings; a pair of exhaust valves for opening and closing the exhaust openings; a valve train for operating the pair of intake valves)and the pair of exhaust valves for opening and closing operation; and a fuel supply device for supplying a liquid fuel into an air intake passage connected to the pair of intake openings; wherein the intake port has a main section, an upper branch section branching from a downstream end of the main section, and a lower branch section branching from the downstream end of the main section and extending below the upper branch section; the pair of intake openings are an upper intake opening of the upper branch section, and a lower intake opening of the lower branch section, respectively; the pair of intake valves are a first intake valve for opening and closing the upper intake opening, and a second intake valve for opening and closing the lower intake opening, respectively; the valve train includes a valve-operation mode changing mechanism capable of changing the respective operating modes of the first and the second intake valve according to engine speed; and the valve-operation mode changing mechanism has a mode selecting system for making the valve-operation mode changing mechanism pause the opening and closing operation of the first intake valve, hold the first intake valve at its closed position and operate the second intake valve for opening and closing operation while the internal combustion engine is operating at engine speeds in a low-speed range.
According to the present invention, while the internal combustion engine included in the outboard motor is operating at engine speeds in a low-speed operating mode, the upper intake opening is kept closed by the paused first intake valve made to pause by the valve-operation mode changing mechanism, and the lower intake opening is opened and closed by the second intake valve to generate swirls in the combustion chamber. Part of the fuel supplied into the intake port flows into the upper branch section and adheres to a wall defining the upper branch section. However, part of the fuel adhering to the wall of the upper branch section collects in a lower part of the upper branch section, and part of the fuel connected in the lower part of the upper branch section flows into the lower branch section and can be easily sucked through the lower intake opening when the second intake valve is opened. Thus, the fuel collected in the upper branch section can be sucked into the combustion chamber more easily than the fuel which may be collected in the lower branch section if the operations of the second intake valve for opening and closing the lower intake opening are made to pause, so that the quantity of the stagnant fuel on the wall of the upper branch section is small.
Thus, the present invention has the following effects. Since the quantity of the stagnant fuel on the wall surface of the upper branch section while the internal combustion engine is operating in a low-speed operating mode in which the operations of the first intake valve are made to pause is reduced, the variation of the air-fuel ratio dependent on the quantity of the stagnant fuel is suppressed and hence stability of combustion is improved. Since swirls are generated to improve combustion, the combustion of a lean air-fuel mixture is possible, which improves fuel consumption rate.
Preferably, an upstream projecting partition wall is formed between the upper branch section and the lower branch section of the intake port. Preferably, the partition wall has inclined upper and lower surfaces.
The valve train may include a first rocker arm for opening and closing the first intake valve, and a second rocker arm for opening and closing the second intake valve, the mode selecting system may interlock the first and the second rocker arm so that the first and the second rocker arm operate synchronously for opening and closing operation while the internal combustion engine is operating at engine speeds in a high-speed range, and the first and the second rocker arms may be disconnected to enable the first rocker arm to pause the opening and closing operation of the first intake valve independently of the operation of the second rocker arm while the internal combustion engine is operating at engine speeds in a low-speed range.
Such operations of the first and the second rocker arm may be achieved by using a mode selecting means which can be set in either a high-speed mode selecting state for a high-speed range for high engine speeds or a low-speed mode selecting state for a low-speed range for low engine speeds according to the engine speed of the internal combustion engine, and an interlocking means capable of interlocking the first and the second rocker arm when the mode selecting means is set in the low-speed mode selecting state.